These seem quite readily available from abroad. My question is that I only brew 12ltr batches and don't mind bottling the odd few litres. How long would beer last in a carbed growler. Looking at the tops with posts for gas.

If you mean the cornelius kegs, the answer depends on a huge number of factors:

Storage temperatureNumber and type of contaminating organismsAlcohol percentageHop loadAmount of yeast carried over (autolysis)General sanitation and purging of the keg with CO2

That said, if I kegged a beer, kept it in the dispensing fridge and it started to turn sour or otherwise went 'off' in six weeks I would be bitterly disappointed. I have beer that is still great two or more months down the line, but in general once I have connected the beer line and started to dispense it I look to finish it within 3 weeks.